This invention relates to protection systems and methods for synchronous machines and more particularly to a method of protecting a synchronous machine arrangement connected to an electricity network against pole-slip and a pole-slip protection relay.
Pole-slipping in synchronous machines occurs when there is insufficient electromagnetic torque to hold the rotor in synchronism with the stator magnetic flux. Typically, pole-slipping occurs under severe fault conditions that cause a transient torque which exceeds the ability of the rotor magnetic field to keep the rotor synchronized with the stator rotating magnetic field. Synchronous generators may pole-slip due to faults in the network, power swings and loss-of-excitation. Synchronous motors may pole-slip during under-voltage conditions, loss of excitation and during sudden mechanical failure.
For example, in the case of a generator connected to a network and driven by a prime-mover in the form of a turbine, in the event of a large fault on the network, the rotor experiences a sudden physical and electrical shift in position relative to the stator. When this occurs, violent acceleration and deceleration forces associated with pole-slipping cause enormous stress on the generator and prime-mover. These extreme stresses may result in damage such as winding movement, shaft fracture and even total destruction.
The known pole-slip protection apparatus and methods make use of an impedance scheme pole-slip protection algorithm to detect pole-slip. These apparatus and methods are reactive in nature in that they first trip a machine after it has pole-slipped one or more times. Severe mechanical damage could be caused by just one pole-slip. To the best of the applicant's knowledge, there is not currently available a pole-slip protection apparatus and method that operate preventatively.